A friction stir welding method is a method of performing welding without adding filler material, by inserting a rotating tool into an unwelded portion of working materials overlapped or butted together, moving the rotating tool while rotating it, and utilizing softening of the working materials caused by frictional heat generated between the rotating tool and the working materials, and the plastic flow created by stirring the softened portions with the rotating tool.
A portion where steel sheets are only butted together and have not been welded yet is referred to as an “unwelded portion”, and a portion where steel sheets have been welded and integrated by a plastic flow is referred to as a “welded portion”.
FIG. 1 shows an example of performing friction stir welding on steel sheets butted together.
As described in FIG. 1, with that friction stir welding method, the edge surfaces of steel sheets 21, 22 are butted together to form an unwelded portion 20, and a rotating tool 10 is moved along the unwelded portion 20 while being rotated. The rotating tool 10 is positioned above the steel sheets 21, 22, and configured to be rotated by a motor (not illustrated). A backing material 30 placed under the steel sheets supports the steel sheets when the steel sheets are pressurized by the rotating tool. The rotating tool 10 which is rotated by driving a motor moves in the direction indicated with arrow F while a probe 12 is kept in contact with the steel sheets 21, 22 at the unwelded portion 20. The probe 12 creates partial regions of plastic material around the probe, and the upper part 11 of a rotating body presses the steel sheets 21, 22 from above to prevent the material from being lost from the plastic zone. In this way, the butted portions of the unwelded portion 20 are heated and softened, and the steel sheets 21, 22 are solid state bonded by the plastic material created by the plastic flow to form a welded portion 25.
Regarding such friction stir welding, studies have been made for a method of performing heating separately from the welding using rotating tools for the purpose of accelerating the welding procedures or reducing welding defects.
For example, JP 3081808 B proposes a welding method using gas flame as the heating device. FIG. 2 shows a schematic diagram describing the procedures of performing friction stir welding using gas flame as the heating device. With that method, various types of gas flame 70 including oxyacetylene, oxypropane, and oxygen natural gas are used as the heat source to heat the welding members 1, 2. With the heating device 72, a substantially cylindrical gas nozzle part 71 from which gas flame 70 is injected is positioned near the front of the probe 62 in the moving direction, and the gas nozzle part 71 moves in conjunction with the movement of the probe 62. Further, the injecting position of the gas flame 70 in the unwelded portion 13 is described as being always positioned ahead of the welding device in the moving direction. Further, the injection width of the gas flame 70 is substantially the same size as the diameter of a shoulder 61 of a rotor 60 when the gas flame strikes the surfaces of the welding members 1, 2 and the tip of the gas flame widens. By doing so, only portions of the unwelded portion 13 near the probe 62 are heated, and the temperature of the portions near the probe and the surrounding regions is raised.
Operation of the heating device when performing the above friction stir welding method will be explained below. While injecting the gas flame 70 from the gas nozzle part 71 of the heating device 72, the rotor 60 of the welding device 3 is rotated and the probe 62 rotating integrally with the rotor 60 is inserted into the unwelded portion 13, and in a state where the probe 62 is inserted, the probe 62 is moved along the butted portion relative to the welding members 1, 2. By doing so, the welding members 1, 2 are welded to form a welded portion 14.
In friction stir welding using the heating device 72 shown in FIG. 2, a goal is to facilitate welding procedures using the probe 62 by heating the unwelded portion with the heat of the gas flame 70 to rapidly soften the contact portion of the probe 62 and the shoulder 61.
In JP 4235874 B, an induction heating device is used as the heat source. According to JP 4235874 B, the time required until initiating friction stir welding is shortened by providing a control mechanism where the temperature up to the temperature where welding is performed by the rotating tool is set to a predetermined temperature and, by doing so, controllability of the heating range and heating temperature is improved and cracks can be prevented from being formed in the welded portion regardless of the material used.
In JP 4537132 B, a laser beam is used as the heat source. According to JP 4537132 B, the unwelded portion is heated before performing welding with a welding tool (which is a rotating tool), and laser beam irradiation is stopped after the unwelded portion reaches a predetermined softening temperature. By doing so, it is described that wear of the welding tool can be suppressed.
As described above, with conventional friction stir welding, techniques of using gas flame, induction heating, or laser beam as the auxiliary heat source during operation have been proposed.
However, when performing friction stir welding of steel sheets using the methods described in JP 3081808 B, JP 4235874 B and JP 4537132 B, embrittlement resulting from temper softening or hardening occurs in the heat-affected zone and sufficient joint strength could not be obtained, even if it is possible to reduce welding defects or accelerate the welding speed.
It could therefore be helpful to provide a friction stir welding method for steel sheets with improved joint strength obtained at a high welding speed without the risk of generating welding defects and damage to the welding tool, or the risk of embrittlement due to temper softening or hardening in the heat-affected zone.